Symptom: When starting game, 6 balls would eject into shooter lane.
Location: Denver, Colorado

Obviously, there was a problem with the ball trough optical sensors.  Upon closer examination, there was no +12V supply voltage on either the IR emitter side of the trough or the receiver side.

To the backbox we go.  The +12V for misc playfield sensors comes off the lower left of the Power Driver board. There are several 4 pin connectors located there (J116, J117, and J118).  All of the connectors looked good.  I checked the +12V there and it was still missing.

Fuse F116 was blown, which supplies the 12 VAC to the rectifier, which generates the DC version of the voltage.